This invention claims priority of a German filed patent application DE 199 49 044.9.
The invention concerns an apparatus for fine positioning of a component, and a coordinate measuring machine having an apparatus for fine positioning of a component.
The article of C. Hoffrogge and H.-J. Rademacher entitled xe2x80x9cA double parallel spring as precision guide systemxe2x80x9d [Eine Doppelparallelfeder als Prxc3xa4zisionsfxc3xchrung], PTB-Mitteilungen 2/73, pp. 79-82, describes an apparatus for fine positioning of a component having an integrally machined double parallel spring. The double parallel spring has a stationary frame and a movable center bar that is retained by multiple parallel springs. Energy transfer from a spindle drive arranged outside the double parallel spring to the center bar is accomplished by way of a coupling member and a wire cable. As a result, the overall system is very large. The double parallel spring described is provided in particular for operation in a horizontal position. Operation in a vertical position requires considerable additional design effort with regard to energy transfer, and the attachment of heavy and therefore also large compensation weights. The overall structure thereby becomes even larger and heavier. A displacement of the center bar with little tilting is possible only by way of a complicated distribution of the applied forces.
The present apparatus for fine positioning of a component is to be used in particular for fine positioning of an objective as it is focused in an ultrahighprecision coordinate measuring machine. A coordinate measuring machine of this kind has an X-Y measurement stage for receiving a substrate whose patterns are to be measured with nanometer precision. The objective with the subsequent imaging optical system is arranged above the X-Y measurement stage. After each change in measurement stage position, e.g. after moving to another pattern that is to be measured, refocusing by positioning of the objective is necessary, since even small irregularities in the measurement stage surface, a minor wedge error in the substrate surface, and slight deflection of the substrate (usually mounted on three support points) will result in a vertical discrepancy in the focal position.
One known measurement method involves using the objective to scan (for example optically) a pattern that is to be measured, and then focusing. The position of the X-Y measurement stage is then measured interferometrically, and the position of the pattern in relation to a reference point is determined from the measurement stage position with nanometer accuracy. Pattern widths or pattern spacings, for example, can then be determined from multiple measurements.
At the desired accuracy, even small sources of error in the position determination must be detected and either eliminated or minimized in the instrument design. One such error source is tilting of the objective with respect to the optical axis of the coordinate measuring machine. This tilting occurs upon vertical movement of the objective during focusing, and results in a lateral offset of the objective axis on the substrate having the patterns to be measured. A pattern to be measured is therefore optically sensed by a tilted objective as being in a different, laterally offset position than it would be by an untilted objective. This discrepancy becomes a measurement error in the measurement result for the position determination of the pattern being measured.
It is therefore an object of the invention to create an apparatus for fine positioning of a component which allows a precise rectilinear movement path for the component that deviates only extremely slightly from a reference axis.
This object is achieved with an apparatus for fine positioning of a component having a one-piece double parallel spring element having an axis of symmetry, including: i) an external stationary frame on whose inner side two holding elements are shaped at positions lying opposite one another about the axis of symmetry; ii) a movable center bar between the holding elements; iii) two movable side bars arranged on either side of the center bar; and iv) pairs of parallel connecting elements, arranged on either side of each holding element and equipped at the ends with bending points, the first connecting element being in each case joined to one side bar and one holding element, and the second connecting element being in each case joined to the side bar and the center bar. An elastic element is located between a holding element and one end of the center bar, and a fine adjustment element is arranged between one holding element and one end of the center bar. The dependent claims recite advantageous embodiments of the invention.
An apparatus according to the present invention for vertical fine positioning of a component has a double parallel spring element that is fabricated in one piece and is configured symmetrically with respect to an axis of symmetry. It comprises a stationary frame that is attached, for example, to a supporting part of a higher-order device. A movable center bar is arranged on the axis of symmetry in the center of the frame. The component to be positioned can be attached to the center bar. Two side bars are movably arranged inside the frame on either side of the center bar.
Two parallel, straight connecting elements are arranged in the region of the ends of each side bar. Bending points in the form of leaf springs are configured at the ends of each connecting element. The first of the two parallel connecting elements creates, by way of the bending points shaped onto it, a movable connection between the side bar and the frame. The second parallel connecting element creates, by way of the bending points shaped onto it, a movable connection between the side bar and the center bar. The center bar is thereby movably connected, with four connecting elements, symmetrically to the two side bars.
The center bar is preloaded in the direction of the frame by way of a spring. The center bar must not touch the frame, since any such contact would cause internal distortion of the double parallel spring element. This would disadvantageously change the bending characteristics of the bending points, and move the center bar out of its movement direction lying strictly parallel to the axis of symmetry. The spring can be configured, for example, as a compression spring or as a tension spring.
Arranged between one end of the center bar and the frame is a fine adjustment element with which the center bar can be adjusted in tilt-free fashion in its longitudinal direction against the spring force of the compression spring. If the adjustment travel of the fine adjustment element is to be not only adjustable but also controllable, it is advantageous to connect the adjustment element to a high-resolution length measurement system for measuring its instantaneous setting. The measured values of the length measurement system are conveyed as the controlled variable to the input of an electronic control system whose output signals control the settings of the fine adjustment element.
A fine adjustment screw, which is driven in either manual or motorized fashion, can be used as the fine adjustment element. In a particularly advantageous embodiment, the fine adjustment element is configured as a piezoelement whose length changes as a function of the magnitude of an applied voltage.
In the apparatus according to the present invention, the configuration of the double parallel spring element is of particular importance. For example, the double parallel spring element must be shaped as a single piece from one plate. The plate must be sufficiently thick that the bending points are configured as wide leaf springs. This is the only way to prevent tilting of the center bar with respect to the axis of symmetry, and to make the double parallel spring element sufficiently stable, in particular in a vertical arrangement. In addition, the plate must be made of a material with a low coefficient of thermal expansion and good flexural characteristics. The low coefficient of thermal expansion ensures that the flexural characteristics of the double parallel spring element remain stable during operation in the presence of small fluctuations in ambient temperature. In an advantageous embodiment the double parallel spring element is made of an Ni36 steel plate.
In the manufacturing process, it is important to ensure that the symmetrically designed configuration of the double parallel spring element is not impaired by excessive production tolerances in material machining, for example cutting, milling, and the like. Any asymmetry in the finished double parallel spring element created in this fashion results in asymmetrical flexural characteristics and in an undesirable tilting movement of the center bar and thus also of the component. It has been demonstrated that shaping of the double parallel spring element using a mechanical machining process generally introduces too much heat into the material, thus causing internal stresses in the material especially at the bending points, and resulting in inhomogeneous flexural characteristics in the individual bending points.
It has further been demonstrated that better and uniform flexural characteristics in the bending points are achieved with electrodischarge machining, which does not heat the material and thus avoids internal stresses in the material. Thus in an advantageous embodiment of the apparatus according to the present invention, the bending points of the double parallel spring element are shaped using an electrodischarge method. In a further and particularly advantageous embodiment of the apparatus according to the present invention, the entire double parallel spring element is shaped using an electrodischarge method. When symmetrical flexural characteristics in the bending points are achieved in this fashion, the springs and the fine adjustment element are arranged in such a way that energy is transferred to the axis of symmetry of the double parallel spring element. This ensures tilt-free movement of the center bar and thus of the component that is to be positioned.
If asymmetries in the flexural behavior of the bending points occur as a result of the manufacturing process, it is advantageous to perform the energy transfer to the spring and the fine adjustment element deliberately outside the axis of the symmetry. With this asymmetrical energy transfer, the asymmetries in flexural characteristics can be compensated for.
The apparatus according to the present invention offers the advantage that it guarantees almost ideal tilt-free movement of the center bar and thus of the component to be positioned. In addition, the apparatus according to the present invention has very compact overall dimensions because the fine adjustment element is arranged inside the frame.
It is a further object of the present invention to create a high-precision measuring instrument having an apparatus for fine positioning of a component, in particular an objective, in which the tilt angle of the objective axis with respect to the strictly vertical optical axis of the high-precision measuring instrument is extremely small. The high-precision measuring instrument is configured, in this context, as a coordinate measuring instrument.
This further object is achieved with a coordinate measuring machine having an apparatus for fine positioning of a component. The coordinate measuring instrument for determining the position, relative to a reference point, of patterns on a transparent substrate includes: a) an incident-light illumination device on a vertical optical axis; b) a frame-shaped measurement stage, displaceable vertically and relative to the optical axis and having an interferometric position determination system, for receiving the substrate; c) an imaging device having an objective that is to be focused, for imaging the patterns of the substrate; d) a vertically arranged one-piece double parallel spring element having an axis of symmetry, comprising: i) an external stationary frame on whose inner side two holding elements are shaped at positions lying opposite one another about the axis of symmetry; ii) a movable center bar between the holding elements; iii) two movable side bars arranged on either side of the center bar; and iv) pairs of parallel connecting elements, arranged on either side of each holding element and equipped at the ends with bending points, the first connecting element being in each case joined to one side bar and one holding element, and the second connecting element being in each case joined to the same side bar and the center bar; e) an elastic element between a holding element and one end of the center bar; f) a fine adjustment element that is arranged between one holding element and one end of the center bar; and g) an objective holder, attached to the center bar, for receiving the objective. The dependent claims recite advantageous embodiments of the coordinate measuring machine.
The apparatus already described for fine positioning is arranged, with the double parallel spring element in a vertical alignment, above the measurement stage of a high-precision measuring instrument, known per se, for measuring a substrate, which has an imaging optical system arranged above it. For that purpose, a stationary supporting part to which the frame of the double parallel spring element is attached is provided above the measurement stage. An objective holder is attached to the center bar of the double parallel spring element.
An objective that has a vertically oriented objective axis is supported by the objective holder. Said axis is, in a neutral position, oriented parallel to the movement direction of the center bar, and aligns with the optical axis of the coordinate measuring instrument. The electronic control system for the fine adjustment drive of the double parallel spring can coact with an autofocus system for automatic focusing of the objective.
The coordinate measuring instrument according to the present invention has the advantage that only extremely small tilt angles of the objective axis with respect to the optical axis of the coordinate measuring instrument occur during fine positioning, i.e. during focusing of the objective. The result on the substrate is therefore a very small lateral offset of the objective axis from the optical instrument axis. With a typical mask, the vertical change in focal position resulting, for example, from its wedge error, deflection, etc. is between 10 and 20 xcexcm. This means a focusing travel for the objective that is also no more than 20 xcexcm, yielding a lateral offset of the objective axis in the coordinate measuring instrument according to the present invention of much less than 1 nm.